To reduce the risk of failure from such condition as mechanical overloading and excessive operating temperatures, most gas turbine engines have limited upper operating speeds.
It is well known in the art of fuel delivery systems for aircraft gas turbine engines to use a fixed positive displacement pump, such as a vane or gear pump, to pressurize fuel for subsequent delivery to the engine. The output of the pump is at an elevated pressure or supply pressure and the inlet of the pump is at a low pressure know as drain pressure. The fixed positive displacement pump provides a flow whose volume is a function of the speed at which the pump is rotating.
The pump output is delivered to a metering valve which in conjunction with a pressure regulating valve, precisely meters the fuel for delivery to the engine. Excess fuel is bypassed from the input of the metering valve to the inlet of the pump or to a fuel reservoir.
Often, overspeed conditions, particularly in such engines employed in powering aircraft, result from the failure of the metering valve in the engine's fuel control, or an erroneous signal from an associated electronic controller which adjusts the metering valve to an excessive high flow setting.
Typical overspeed systems monitor the speed of the aircraft turbine and upon detection of an overspeed condition will either stop the flow of fuel to the engine thus shutting it down, or reduce the flow of fuel to the engine, thus operating the engine at a reduced speed.
The typical overspeed governor consists of a spool valve, which controls various pressure signals, the position of which is determined by turbine speed. At system start, the spool of the overspeed governor will be at an initial position and will translate as turbine speed increases. When an overspeed condition is sensed, the overspeed governor will cause a shutdown of the engine or reduce fuel flow to cause the engine to operate at a reduced speed.
As part of preflight check-out of an aircraft, the overspeed system is tested to ensure proper function. If the overspeed system fails, the aircraft is prevented from taking off until the failure is diagnosed. The check out of the overspeed system is performed using a pressure switch for monitoring the various pressure signals controlled by the overspeed governor or by a microswitch for monitoring the position of the spool. These switches are typically monitored by the electronic engine control (EEC). If the EEC fails to detect switch closure at a given turbine speed, the EEC shuts the engine down.
The switches used for check out of the overspeed governor typically have a higher failure rate than the overspeed governor. This leads to situations where the switch indicates an overspeed system failure, when in fact only the switch has failed. However, due to safety concerns the aircraft must remain grounded until the failure has been diagnosed and corrected.
Therefore, there exists a need for an overspeed system with increased reliability and built in test capability.